Disinfection, as applied in water and wastewater treatment, is a process by which pathogenic microorganisms are inactivated to provide public health protection. Chlorination has been the dominant method employed for disinfection for almost 100 years. However, it is no longer the disinfection method automatically chosen for either water or wastewater treatment because of the potential problems associated with disinfection by-products and associated toxicity in treated water. UV irradiation is the most frequently chosen alternative to conventional chlorination. Since UV radiation is a nonchemical agent, it does not yield any disinfectant residual. Therefore, the concerns associated with toxic disinfectant residuals do not apply. In addition, UV disinfection is a rapid process. Little contact time (on the order of seconds rather than minutes) is required. The result is that UV equipment occupies little space when compared to chlorination and ozonation.
The vast majority of UV disinfection systems employ an open-channel configuration. Two types of open-channel UV disinfection systems exist: horizontal systems, in which ultraviolet radiation-producing lamps, hereinafter referred to as "lamps," are arranged parallel to the direction of flow; and vertical systems, in which lamps are perpendicular to the water surface. For most vertical UV systems, the lamps are arranged in a staggered pattern to promote mixing.
The responses of microorganisms to UV irradiation are entirely attributable to the dose of radiation to which they are exposed. The UV dose is defined as the product of radiation intensity and exposure time. As a result of turbulent flow conditions and three-dimensional spatial variations in UV intensity, continuous-flow UV systems deliver a broad distribution of UV doses. The elementary principles of reactor theory can be used to demonstrate that this distribution of doses leads to inefficient use of the UV energy emitted within these systems. Furthermore, the theoretical upper limit on UV reactor performance coincides with a system which accomplishes the delivery of a single UV dose (i.e., a dose distribution which can be represented by a delta function). Optimal dose distribution is not possible in currently used UV disinfection systems.
An average dose does not accurately describe the disinfection efficiency of a full-scale UV system. The UV intensity is a function of position. The intensity of the UV radiation decreases rapidly with distance from the source of radiation. The exposure time is not a constant either. The complex geometry of open-channel UV systems dictates complex hydrodynamic behavior as well, with strong velocity gradients being observed. Coincidentally, the velocity is generally highest in areas of lowest intensity. This creates a situation in which some microorganisms are exposed to a low UV intensity over a comparatively short period of time, thereby allowing them to "escape" the system with a relatively low UV dose. This represents a potentially serious process limitation in open-channel UV systems. For example, if 1% of the microorganisms received doses lower than the lethal level, then the maximum inactivation which could be achieved by the system would be 99%, no matter what actual average dose was delivered.
Due to the lack of information regarding hydrodynamic behavior in UV systems, there has been no acceptable approach by which disinfection efficiency of any open-channel UV reactor can be accurately predicted. Therefore, design decisions have been based on empirical observations and past experience. Furthermore, the non-uniform distribution of UV doses in these systems indicates that UV radiation is applied inefficiently. While UV overdose apparently presents no danger in terms of finished water composition, it does increase operating and capital costs. Therefore, it is desired to have a system which incorporates the effects of hydrodynamic behavior and the UV intensity field to provide for complete disinfection.